1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device having a wide viewing angle which is suitable for use in a flat display in a personal computer, a word processor, an amusement apparatus or a TV, or in a display apparatus utilizing a shutter effect.
2. Description of the Related Art
Known techniques for improving or widening the viewing angle of a liquid crystal display device include: moving the liquid crystal molecules only in a direction substantially parallel to the substrate surface; and dividing one pixel into a plurality of regions respectively having different orientations of liquid crystal molecules while the liquid crystal molecules move in a direction vertical to the substrate surface. A representative display mode which uses the former technique is the IPS (In-Plane Switching) mode. Representative display modes which uses the latter technique include: a wide viewing angle liquid crystal display mode (Japanese Laid-Open Publication No. 7-120728) where an Np (Nematic positive) type liquid crystal material is in an axially symmetric horizontal orientation; another wide viewing angle liquid crystal display mode (Japanese Laid-Open Publication No. 7-28068) where a vertically-oriented Nn (Nematic negative) type liquid crystal material is divided into regions and oriented differently in different regions by controlling an electric field applied therethrough; and still another wide viewing angle liquid crystal display mode (disclosed in AM-LCD""96, p. 185 (1996)) where an Np (Nematic positive) type liquid crystal material in each pixel is divided substantially equally into four regions, with the liquid crystal material being in different parallel orientations in each region.
Generally, in the latter technique, where each pixel is divided into a plurality of regions having liquid crystal molecules of different orientations, the viewing angle characteristic along an axial direction which extends equiangularly between a polarization axis 202 of an upper polarizing plate and an absorption axis 203 of a lower polarizing plate (which interpose a liquid crystal cell 201 therebetween as illustrated in FIG. 23B) is considerably inferior to the viewing angle characteristic along one of the absorption axes. Referring to FIG. 23A, a polar coordinate system is defined where: xcex8 represents an angle from a direction normal to an imaginary plane 204 parallel to the liquid crystal cell to a viewing direction 205 from which the viewer is viewing the display; and "PHgr" denotes an azimuth of the viewing direction 205 with respect to the absorption axis 203 (where "PHgr"=0xc2x0) of a lower polarizing plate. When the viewing angle characteristic is evaluated in such a polar coordinate system, an isocontrast contour curve, as illustrated in FIG. 24, results generally irrespective of the display mode. Thus, the viewing angle narrows as the azimuth "PHgr" shifts from the absorption axis of the upper or lower polarizing plate. A curve 302, also illustrated in FIG. 24, represents an isocontrast contour curve which the present invention aims to obtain.
The inventors of the present invention disclosed (Japanese Laid-Open Publication No. 7-120728) an ASM mode (Axially Symmetric Aligned Microcell Mode) where the liquid crystal molecules are oriented in an axially symmetric pattern in each pixel. In this mode, the phase separation of a mixture of a liquid crystal material and a photocurable resin is used to orient the liquid crystal molecules in an axially symmetric pattern. The Np type liquid crystal material used in this mode is such that the axially symmetric liquid crystal molecules are oriented vertically to the substrate by applying a voltage thereto.
A liquid crystal display device of this conventional ASM mode uses a liquid crystal material whose dielectric anisotropy AE has a positive value. This display mode exhibits excellent display characteristics in any direction since the liquid crystal molecules are oriented in axial symmetry. However, when the respective absorption axes of the polarizing plates are arranged in a crossed-Nicols arrangement, the viewing characteristic tends to deteriorate. Moreover, a black matrix having a large area is required for preventing light from leaking in an OFF state. Furthermore, this conventional ASM mode utilizes a phase separation process which requires a complicated temperature control in order to realize the axially symmetric orientation. Therefore, a liquid crystal display device of this ASM mode is difficult to produce and the obtained axially symmetric orientation is unstable which is not reliable, particularly at a high temperature.
In order to solve such problems, the inventors of the present invention proposed (Japanese Laid-Open Publication No. 8-341590) a liquid crystal display device where the liquid crystal molecules are oriented in axial symmetry in each pixel region, which thus has a high contrast and exhibits excellent display characteristics in any azimuth, and a production method which allows such a liquid crystal display device to be produced easily.
The liquid crystal display device has: a pair of substrates interposing a liquid crystal layer therebetween including liquid crystal molecules with a negative dielectric anisotropy (xcex94∈ less than 0); and a vertical alignment layer on one surface of each substrate adjacent to the liquid crystal layer. Protrusions are further provided on at least one of the substrates so as to surround each pixel region. Moreover, a pair of polarizing plates are provided between the pair of substrates so that the respective absorption axes of the polarizing plates are perpendicular to each other. This liquid crystal display device does not require a complicated production process, but still realizes such an orientation where the liquid crystal molecules are oriented substantially perpendicular to the pair of substrates in the absence of an applied voltage, and are axially symmetric in each pixel region in the presence of an applied voltage.
In this liquid crystal display device, in the absence of an applied voltage, the liquid crystal molecules are oriented substantially perpendicular to the pair of substrates, thereby realizing a satisfactory black state and thus a high contrast display in a viewing angle normal to the display plane. From different viewing angles, however, light leakage is observed, and the contrast ratio is deteriorated, because (i) some of the viewing angle dependency results from an inherent characteristic of the polarizing plate, and (ii) the retardation value of the vertically-oriented liquid crystal molecules varies from one direction to another, thereby causing the retardation value of the liquid crystal layer to have a viewing angle dependency.
Hereinafter, the viewing angle dependency resulting from an inherent characteristic of the polarizing plate will be explained. When light is incident upon the above-described wide viewing angle mode liquid crystal display device from a direction of the polarization axis (transmittance axis) of the polarizing plate and passes through the refractive index ellipsoid of the liquid crystal layer, then, such light only contains a normal light component or an abnormal light component. However, when light is incident upon the device from a direction shifted by 45xc2x0 from the absorption axis of the polarizing plate and passes though the refractive index ellipsoid, then, such light contains both normal and abnormal light components and thus is elliptically-polarized light. In such a case, apparent light leakage increases as the direction of the vibration of the polarized light shifts from one of the absorption axes of the polarizing plates, which are perpendicular to each other.
Hereinafter, the viewing angle dependency resulting from the varying retardation value of the liquid crystal layer will be explained. In the above-described liquid crystal display device, the liquid crystal molecules are oriented substantially vertically to the pair of substrates in the absence of an applied voltage. Accordingly, the retardation value varies depending upon the direction from which the display is viewed, whereby the viewing angle dependency is observed.
The viewing angle characteristic is particularly poor in a direction at about 45xc2x0 from both of the absorption axes of the polarizing plates which are perpendicular to each other. The poor viewing angle characteristic occurs because in such a direction the inherent characteristic of the polarizing plate and the varying retardation value both affect the viewing angle characteristic of the display device. For example, in a direction at about 45xc2x0 with respect to the absorption axis of the polarizing plate, the contrast of the display device considerably deteriorates over a certain viewing angle range, e.g., from about 35xc2x0 to about 50xc2x0, where the gray-scale level is inverted. Thus, the display characteristic greatly deteriorates particularly in a gray-scale display.
According to one aspect of this invention, a liquid crystal display device includes: a liquid crystal cell having a pair of substrates and a liquid crystal layer interposed therebetween; a pair of polarizing plates interposing the liquid crystal cell therebetween; and a phase compensation element provided between at least one of the polarizing plates and the liquid crystal cell. A refractive index anisotropy value of the liquid crystal layer along a plane parallel to a surface of the liquid crystal cell is smaller in a black display than in a white display. The phase compensation element has three principal refractive indices nx, ny and nz respectively along x, y and z axes thereof which are perpendicular to one another, and when nx and ny are the principal refractive indices along a plane parallel to a surface of the liquid crystal cell with nz being the principal refractive index along a thickness direction of the liquid crystal cell, wherein the x axis is parallel to an absorption axis of one of the polarizing plates closer to a viewer viewing the liquid crystal display device, and the principal refractive indices nx, ny and nz satisfy the following expressions: nz less than (nx+ny)/2; and nxxe2x89xa0ny.
In one embodiment of the invention, first and second phase compensation elements are provided respectively between one of the polarizing plates and the liquid crystal cell and between the other one of the polarizing plates and the liquid crystal cell. The first and second phase compensation elements each have a maximum refractive index axis along which the phase compensation element exhibits a maximum refractive index in the plane parallel to the surface of the liquid crystal cell, the axes being perpendicular to each other.
In one embodiment of the invention, the maximum refractive index axis of each of the first and second phase compensation elements is perpendicular to an absorption axis of one of the polarizing plates which is adjacent to the phase compensation element.
In one embodiment of the invention, the liquid crystal layer includes a nematic liquid crystal material which has a negative dielectric anisotropy, where liquid crystal molecules of the nematic liquid crystal material are oriented substantially perpendicular to the substrate in an absence of an applied voltage.
In one embodiment of the invention, the liquid crystal layer includes a nematic liquid crystal material which has a positive dielectric anisotropy, where liquid crystal molecules of the nematic liquid crystal material are oriented substantially parallel to the substrate in an absence of an applied voltage.
In one embodiment of the invention, the liquid crystal cell includes a plurality of pixel regions, each of the pixel regions including two or more liquid crystal regions, respectively, having different orientations of liquid crystal molecules.
In one embodiment of the invention, the liquid crystal cell includes a plurality of pixel regions, and an orientation of liquid crystal molecules varies continuously in each of the pixel regions.
In one embodiment of the invention, a birefringence xcex94n of the liquid crystal molecules, an average thickness dLC of the liquid crystal layer, and a thickness df of the phase compensation element satisfy the following expressions: 0xe2x89xa6{dfxc2x7(nxxe2x88x92ny)}/(dLCxc2x7xcex94n)xe2x89xa60.12; and 0.05xe2x89xa6{dfxc2x7(nxxe2x88x92nz)}/(dLCxc2x7xcex94n)xe2x89xa60.69.
In one embodiment of the invention, the liquid crystal layer includes a plurality of liquid crystal regions. The pair of substrates each have a vertical alignment layer on one surface thereof which is closer to the liquid crystal layer, and the liquid crystal molecules are oriented in axial symmetry in each of the liquid crystal regions in a presence of an applied voltage. Respective absorption axes of the pair of polarizing plates are perpendicular to each other. The phase compensation element has a negative birefringence and a relationship nx greater than ny greater than nz.
In one embodiment of the invention, the phase compensation element is provided between the liquid crystal cell and each of the polarizing plates.
In one embodiment of the invention, the phase compensation element includes a biaxial film having retardations along an in-plane direction and along a thickness direction, respectively, or a layered film obtained by attaching together a uniaxial film having a retardation along the in-plane direction and a uniaxial film having a retardation along the thickness direction.
In one embodiment of the invention, the x axis of the phase compensation element is substantially perpendicular to an absorption axis of one of the polarizing plates which is adjacent to the phase compensation element.
In one embodiment of the invention, an angular shift between the x axis of the phase compensation element and a direction perpendicular to the absorption axis of the polarizing plate is equal to or less than about 1xc2x0.
In one embodiment of the invention, a retardation value dfxc2x7(nxxe2x88x92ny) of the phase compensation element along an in-plane direction thereof is less than a retardation value dLCxc2x7xcex94n of the liquid crystal layer, where: xcex94n denotes a birefringence of the liquid crystal molecules; dLC denotes an average thickness of the liquid crystal layer; and df denotes a thickness of the phase compensation element.
In one embodiment of the invention, the retardation value dLCxc2x7xcex94n of the liquid crystal layer is in a range of 300 nm to 550 nm.
In one embodiment of the invention, the birefringence xcex94n of the liquid crystal molecules, the average thickness dLC of the liquid crystal layer and the thickness df of the phase compensation element satisfy the following expressions: 0xe2x89xa6{dfxc2x7(nxxe2x88x92ny)}/(dLCxc2x7xcex94n)xe2x89xa60.13; and 0xe2x89xa6{dfxc2x7(nxxe2x88x92nz)}/(dLCxc2x7xcex94n)xe2x89xa60.72.
In one embodiment of the invention, a retardation value dfxc2x7(nxxe2x88x92nz) of the phase compensation element along a thickness direction thereof is less than a retardation value dLCxc2x7xcex94n of the liquid crystal layer, where: xcex94n denotes a birefringence of the liquid crystal molecules; dLC denotes an average thickness of the liquid crystal layer; and df denotes a thickness of the phase compensation element.
In one embodiment of the invention, the retardation value dLCxc2x7xcex94n of the liquid crystal layer is in a range of 300 nm and 550 nm.
In one embodiment of the invention, the birefringence xcex94n of the liquid crystal molecules, the average thickness dLC of the liquid crystal layer and the thickness df of the phase compensation element satisfy the following expressions: 0xe2x89xa6{dfxc2x7(nxxe2x88x92ny)}/(dFCxc2x7xcex94n)xe2x89xa60.13; and 0xe2x89xa6{dfxc2x7(nxxe2x88x92nz)}/(dLCxc2x7xcex94n)xe2x89xa60.72.
In one embodiment of the invention, the phase compensation element is such as to satisfy the following expression: 0.035xe2x89xa6{dfxc2x7(nxxe2x88x92ny)}/(dLCxc2x7xcex94n)xe2x89xa60.15.
In one embodiment of the invention, a retardation value dfxc2x7(nxxe2x88x92nz) of the phase compensation element along a thickness direction thereof is greater than 0.
In one embodiment of the invention, a ratio between a retardation value dfxc2x7(nxxe2x88x92ny) of the phase compensation element along an in-plane direction thereof and the retardation value dfxc2x7(nxxe2x88x92nz) of the phase compensation element along the thickness direction thereof is equal to or greater than 2.
In one embodiment of the invention, the ratio between the retardation value dfxc2x7(nxxe2x88x92ny) of the phase compensation element along the in-plane direction thereof and the retardation value dfxc2x7(nxxe2x88x92nz) of the phase compensation element along the thickness direction thereof is in a range of about 3 to about 6.
In one embodiment of the invention, an average refractive index of the phase compensation element is in a range of about 1.4 to about 1.7.
In one embodiment of the invention, an antiglare layer is provided on a surface of the one of the polarizing plates closer to the viewer viewing the liquid crystal display device.
In one embodiment of the invention, an antireflection film is provided on a surface of the antiglare layer.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device having a generally axially symmetric viewing angle characteristic in which the viewing angle characteristic is prevented from deteriorating as the viewing direction shifts from the absorption axis.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.